The basic structural features of an axial-flow gas turbine engine are well known. In brief, the engine comprises a compressor section, combustor section and turbine section arranged longitudinally around the engine centerline so as to provide an annular gas flow path. The compressor section compresses incoming atmospheric gases that are then mixed with a combustible fuel product and burned in the combustor section to produce a high energy exhaust gas stream. The turbine section extracts power from the exhaust gas stream to drive the compressor section. The exhaust gas stream produces forward thrust as it rearwardly exits the turbine section. Some engines may include a fan section, which is also driven by the turbine section, to produce by pass thrust.
Both the compressor and the turbine section include an inner rotor having a plurality of blades extending substantially radially outwardly therefrom and arranged in groups of circumferential rows. The rows of rotor blades are interdigitated with radially inwardly extending rows of blades attached to an outer engine casing. Depending upon the particular gas turbine engine involved, the inwardly extending blades may also rotate.
In general, the compressor or turbine rotor comprises a generally cylindrically shaped spool having a plurality of webs extending radially inwardly from the inner surface of the spool. The webs each terminate in an annular thickened portion known as a disk, leaving a circumferential opening at the center thereof. Effectively, these openings form the bore of the rotor through which the engine drive shafts extend.
Because of the great rotational speeds of the rotors, it is important to balance the rotors to minimize engine vibrations. To this end, engine manufacturers strive to remove any excess material that may unbalance the rotors. Additionally, because of the fact that increasing engine weight decreases engine efficiency, it is important for engine manufacturers to remove as much unneeded material as possible from the engine parts. In particular, where engine parts are welded together, such as rotor sections that are joined by the inertia welding, electron beam welding, laser welding, or other forms of materials joining processes, it is incumbent upon the manufacturer to remove welding flash that is created during the materials joining operation from both the inner and outer spool surfaces.
Commonly, the welding flash is removed from the inner spool surface by conventional machining techniques wherein a machining means is moved into a working relationship with a rotating working surface. That is, a machining or surface contouring tool having an elongate mounting post with a tool holder attached to the end of the post is inserted into the bore of the rotating rotor. The tool holder has a machining insert disposed at the end thereof such that the machining post and tool holder/insert together have a generally "L" shaped configuration. The tool is then moved so that the insert is in a working position relative to the working inner rotor surface.
It is mechanically advantageous for an engine to have a large internal rotor or spool diameter d relative to the bore diameter b. This ratio is in part dependent upon the ability of the available machining tools to work the surfaces to the desired contours. With currently available machining tools it can be shown that the relationship between these two quantities is defined by the following equation: EQU d.sub.max =3b-2t,
where d.sub.max equals the maximum internal rotor surface diameter that can be machined by currently available machining or contouring tools; b=the bore diameter; and t=the thickness of the tool mounting post.
Because of the mechanical advantage of a large spool diameter to bore diameter ratio, it would be desirable to have a machining tool having an increased reach so that the ratio d.sub.max /b can be increased over that presently achievable with currently available machining tools. Such an improved surface contouring tool would enable engine manufacturers to build gas turbine engines where rotor dimensions are dictated by engine operating dynamics rather than by manufacturing limitations.